For high precision length measurements in coordinate measurement technology, it is necessary that the temperature of the workpiece be detected in the course of the measurement in order that length changes of the workpiece caused by temperature can be computer corrected. This becomes evermore significant as the use of coordinate measuring apparatus is shifted more and more from highly climatized fine measurement rooms into manufacturing areas where large temperature fluctuations are possible.
The workpiece temperatures are detected with contact thermometers which are applied to the object to be measured. Generally, these contact thermometers are precision resistors which are applied to the workpiece utilizing various attachment devices. In addition to manually applied contact sensors, U.S. Pat. No. 5,011,297 discloses an automatic version of detecting the temperature of workpieces.
U.S. Pat. No. 5,011,297 describes the exchange of the coordinate measuring probe for a temperature sensor component with the aid of a probe changing system. This temperature sensor is resiliently biased against the workpiece with the aid of the measuring arm of the coordinate measuring apparatus and maintains the contact to the workpiece for a time and thereafter supplies the determined temperature measurement values to the computer of the coordinate measuring apparatus. In this method, a check is made by means of a plausibility control of the detected measurement values as to whether the sensor is actually in contact with the object to be measured. However, no temperature change at the sensor takes place for the same sensor temperature and workpiece temperature so that no definite answer can be obtained as to whether reliable contact has been made between the sensor and the object. A further difficulty is in the fixing of the measurement duration, that is, the contact time during which the sensor is in contact engagement with the workpiece. For large temperature differences between sensor and workpiece, more time is needed until the sensor assumes the temperature of the workpiece. Accordingly, in order to be certain that the actual workpiece temperature has been detected, it is necessary to set longer measurement intervals which negatively affects the desired short measurement times.